The present invention relates to an external resonator type semiconductor laser in which a laser beam emitted from the semiconductor laser is adapted to be fed back to the same by means of an external reflection member (mirror).
The oscillatory axial mode in a semiconductor laser has here to for been selected according to gain distribution in the laser medium and the transparency characteristic of the laser resonator. FIG. 7 is a diagram schematically showing selectivity of the oscillatory axial mode in a conventional semiconductor laser, FIG. 7(a) showing gain distribution in the laser medium against wavelength (the abscissa), FIG. 7(b) showing a spectrum of the axial modes against wavelength, and FIG. 7(c) showing a spectrum in a superradiant state in which the above graphs (a) and (b) are superposed on each other. Of the axial modes in the laser, the one having the wavelength closer to the peak (the maximum value) in the gain distribution obtains the maximum gain and becomes the oscillatory axial mode. However, when the environmental temperature changes, the wavelength of the peak in the gain distribution shifts toward the higher wavelength side at the rate of 2-3 .ANG./deg because the band gap in the semiconductor is varied. Meanwhile, the refractive index of the medium is varied and also the laser element itself exhibits thermal expansion thereby changing the effective optical length of the laser resonator. As a result, the axial modes shift toward the higher wavelength side at the rate of approximately 0.7 .ANG./deg, maintaining approximately 3 .ANG. of spacings therebetween. Thus, as the temperature rises from a point, since the amount of the variation of the gain distribution is larger than the amount of the variation of the axial modes, the oscillatory wavelength shows a continuous variation for a while but soon it exhibits a mode hopping, and thereafter, it repeats continuous variation and mode hopping, and thereby, stepped variations as shown in FIG. 9 are caused. Besides, the wavelength is also changed by the magnitude of the current for driving the semiconductor laser. Such variations have hitherto been hindrances to development of the applications of the semiconductor lasers to uses such as a laight source for wavelength-multiplex optical communications or high-resolution spectroscopy of which a bright future is expected.
A SEC. laser (Short External Cavity Laser diode) has been invented which is adapted such that an emitted beam from the semiconductor laser is fed back by means of an external mirror to the main body of the semiconductor laser, and in this arrangement, the oscillatory axial mode is selected according to three factors, i.e., the ordinary laser gain distribution, the laser axial mode, and wavelength selectivity by the external resonator. The relative situation is schematically shown in FIG. 8 in correspondence with FIG. 7. FIG. 8(a) shows the gain distribution in the laser medium against wavelength (the absciosa), FIG. 8(b) shows a spectrum of the axial modes against wavelength, FIG. 8(c) shows a resonance characteristic of the external resonator against wavelength, and FIG. 8(d) shows the spectrum in a superradiant state in which the above (a), (b), and (c) are superposed. The envelope of the spectrum in the superradiant state, is different from that in FIG. 7, and has ripples as shown in FIG. 8(d). In this case, since the temperature characteristic of the peak of the envelope is controllable by changing the length of the external resonator, i.e., the length of the gap between the semiconductor laser and the external mirror, it becomes possible to suppress the mode hopping.
Typical examples of characteristics against temperature of the oscillatory wavelength of the SEC laser are shown in FIGS. 10(a), (b), and (c), in any of which the same axial modes are maintained within the temperature range .DELTA.t, whereas, within the temperature range .DELTA.T, the axial modes in the same mountain in the envelope of the spectrum as shown in FIG. 8(d) each successively acquire the maximum gain to become the oscillatory axial mode, and when the range .DELTA.T is exceeded, the oscillatory axial mode shifts to the peak of the next mountain in the envelope and thereupon exhibits a large mode hop. As to d.lambda./dT representing the temperature coefficient of the wavelength of the peak of the envelope as shown in FIG. 8(d) and .alpha. representing the temperature coefficient of the axial modes as shown in FIG. 8(b), FIG. 10(a) shows such a state when d.lambda./dT&lt;.alpha. of the oscillatory axial mode successively shifting to the adjoining axial mode on the side toward shorter wavelengths and producing small mode hops within the range .DELTA.T, whereas FIG. 10( b) shows such a state when d.lambda./dT =.alpha., then .DELTA.T becoming .DELTA.T=.DELTA.t, of the same only producing a large mode hop. Further, FIG. 10(d) shows such a state when d.lambda./dT &gt;.alpha. of the oscillatory axial mode successively shifting to the adjoining axial mode on the side toward longer wavelengths and producing small mode hops.
The conventional SEC laser is constructed of a VSIS type semiconductor laser having a GaAs.GaAlAs.DH (double hetero) structure on a GaAs substrate and a GaAs chip provided with an Al.sub.2 O.sub.3 coating serving as a total reflection mirror, both being fixed on a mount made of Cu at a predetermined external resonator distance (the distance between the emitting end surface of the semiconductor laser and the reflecting mirror surface). An example of the relationship between the external resonator distance L of the SEC laser and the temperature range .DELTA.T, as shown in FIGS. 10(a), (b), and (c), is indicated by a curve (C) in FIG. 4. And at this time, the temperature coefficient d.lambda./dT of the wavelength of the peak of the envelope of the spectrum has a characteristic against L as shown by a curve C in FIG. 5. From these FIGS. .DELTA.T&lt;35.degree. C. is obtainded for L&gt;50 .mu.m and .DELTA.T&gt;35.degree. C. is obtained for L&lt;50 .mu.m, but when .DELTA.T &gt;35.degree. C., the variation of the oscillatory wavelength becomes as large as shown in FIG. 10(c). Therefore, to widen the range .DELTA.T and reduce the variation of the oscillatory wavelength in such case, it has so far been practiced to set d.lambda./dT to agree with the temperature coefficient of the axial modes so that a characteristic as shown in FIG. 10(b) is obtained.
Therefore, when the setting is made to L=50 .mu.m to make d.lambda./dT coincident with the temperature coefficient (0.7 .ANG./deg) of the axial modes, it is achieved to suppress the mode hopping over the temperature range .DELTA.T=35.degree. C., but the variation of the oscillatory wavelength in the range .DELTA.T becomes as large as 35.degree. C..times.0.7 .ANG./.degree.C.=24.5 .ANG..